The environmental remediation industry needs more effective technologies for light non-aqueous phase liquid (LNAPL, including residual phase LNAPL) remediation and the associated dissolved-phase of benzene, toluene, ethylbenzene, and xylenes (BTEX) plumes, especially where the aerial extent of contamination is large and not practicable to remediate using existing technologies. LNAPL recovery by extracting fluids via pumping and/or other means, recovers a portion of the LNAPL with the remaining residual LNAPL being held by capillary forces in soil porosity. The long-term pumping of fluids reaches a point of diminishing LNAPL recovery relative to the costs required to maintain recovery systems. The remaining LNAPL often serves as a continuing source of dissolved hydrocarbon plumes including BTEX and low molecular weight poly-aromatic hydrocarbons (or polynuclear aromatic hydrocarbons) (PAH) such as naphthalene. Cost effective approaches to remove or degrade residual LNAPL do not exist. Large areas of residual LNAPL therefore remain at numerous sites including terminals and refineries across the world. Existing technologies used to enhance the recovery of the remaining LNAPL include excavating the impacted soils, surfactant and/or solvent flooding, various in-situ heating approaches (steam flooding, electrical resistance heating, among others) and air-sparging. These approaches are costly and often not reasonable to apply over broad areas of LNAPL impacted soil, nor practical at some facilities with extensive build-out or development. Existing technologies used to degrade LNAPL in-situ include chemical oxidation injections and burning LNAPL below ground; the former is costly over larger areas and the latter is in an experimental stage.
Enhancing the aerobic biodegradation of LNAPL is limited by the low solubility of dissolved oxygen and difficulty in distributing oxygen in the subsurface. Anaerobic conditions predominate in subsurface hydrocarbon impacted soils and groundwater, especially within LNAPL source zones. In the absence of oxygen, the anaerobic biodegradation of a wide variety of hydrocarbons is known to occur with varied soluble electron acceptors including nitrate and sulfate, with insoluble ferric iron and manganese oxides, and under methanogenic conditions where electron acceptors other than carbon dioxide are depleted.
The application of enhanced in-situ anaerobic bioremediation has been limited to treating dissolved plumes. Sulfate and/or nitrate have been introduced as electron acceptors to enhance the anaerobic bioremediation of dissolved BTEX plumes but not specifically LNAPL. U.S. Pat. No. 7,384,556 to Song et al. claims the use of “sulfur” and other nutritional sources for stimulating biodegradation in aquifers impacted with both LNAPL and DNAPL but does not indicate what form of sulfur to use, the necessary quantities of nutrients required for LNAPL biodegradation, techniques to administer the necessary quantities, how to target LNAPL impacted zones, or how to sustain the anaerobic biodegradation of LNAPL.
Currently in the bioremediation field, limited biodegradation of non-aqueous phase liquids is thought to occur due to hydrocarbon solubility constraints and microorganisms requiring an aqueous phase environment to thrive. Efforts to stimulate anaerobic hydrocarbon biodegradation have been limited to degrading aqueous phase hydrocarbons or by enhancing LNAPL hydrocarbon solubility and biodegradation using surfactants or cosolvents. U.S. Pat. No. 6,720,176 to Hince et al. provides a method to enhance the anaerobic biodegradation of hydrocarbons using a surfactant and chelating agents with a sulfate containing compound and a source of phosphate. No. 2010/0227381 to Hoag and Collins provides a method to enhance the aerobic biodegradation of LNAPL through the combined use of surfactants or cosolvents with a chemical oxidant to partially oxidize solubilized or desorbed hydrocarbons into more readily biodegradable compounds. The use of surfactants or chemical oxidants greatly increases remediation costs and can be associated with negative consequences. Surfactant addition has the potential for mobilizing hydrocarbons outside of the treatment areas, and varying results including the inhibition of biodegradation are often observed.
U.S. Pat. No. 6,787,034 to Noland and Elliott provides a method to accelerate the anaerobic biodegradation of hydrocarbons using a mixture of an adsorbent capable of adsorbing hydrocarbons, anaerobic bacteria, a sulfate-containing compound that releases sulfate over time, and a sulfide scavenging agent. The reference does not address how to accelerate the biodegradation of LNAPL as indicated based on adding a hydrocarbon adsorbent for lower concentrations of hydrocarbons. In addition, adding anaerobic bacteria capable of biodegradation hydrocarbons under sulfate-reducing conditions is not necessary as they are ubiquitous.
Therefore, there is a need for a method to stimulate and sustain the anaerobic biodegradation of LNAPL (including residual LNAPL) by enhancing the anaerobic biodegradation of LNAPL using indigenous anaerobic microorganisms to reduce or eliminate the toxicity and ecological threat of LNAPL released into the subsurface.